U.S. patent application number 13/975789 was filed with the patent office on 2015-02-26 for apparatus and method of silent monitoring alarm sounders.
The applicant listed for this patent is Honeywell International Inc.. Invention is credited to Michael BARSON.
Application Number | 20150055786 13/975789 |
Document ID | / |
Family ID | 51301134 |
Filed Date | 2015-02-26 |
United States Patent
Application |
20150055786 |
Kind Code |
A1 |
BARSON; Michael |
February 26, 2015 |
Apparatus and Method of Silent Monitoring Alarm Sounders
Abstract
An alarm sounder, which incorporates a piezo-electric output
transducer, can be silently monitored using a variable frequency
square wave. An initial frequency, close to the upper limit of
human hearing, is coupled to the sounder. The transducer draws very
little current at this initial frequency. The frequency of the
square wave is systematically reduced, and the current draw is
continually monitored. A high current indicates a low impedance
type of fault. A low current throughout the frequency range
indicates a potential high frequency type of fault.
Inventors: |
BARSON; Michael; (Nuneaton,
GB) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Honeywell International Inc. |
Morristown |
NJ |
US |
|
|
Family ID: |
51301134 |
Appl. No.: |
13/975789 |
Filed: |
August 26, 2013 |
Current U.S.
Class: |
381/59 ;
381/120 |
Current CPC
Class: |
H04R 17/00 20130101;
H03F 3/217 20130101; H04R 29/001 20130101; G08B 29/126 20130101;
G10K 9/122 20130101 |
Class at
Publication: |
381/59 ;
381/120 |
International
Class: |
G08B 29/10 20060101
G08B029/10; H04R 17/00 20060101 H04R017/00; H03F 3/217 20060101
H03F003/217; H04R 29/00 20060101 H04R029/00 |
Claims
1. An alarm sounder comprising: a housing; a class-D amplifier
coupled to a piezo-electric transducer, the class-D amplifier and
the piezo-electric transducer are carried in the housing; and
control circuits coupled to the class-D amplifier that incorporate
silent testing of the alarm sounder by coupling a monitoring
waveform to the class-D amplifier at an initial, selected high
frequency, with a low frequency signal envelope, substantially
inaudible to human hearing and a plurality of descending
frequencies with the transducer current being measured at each of
the frequencies.
2. The alarm sounder as in claim 1 which communicates with a
displaced regional monitoring control unit.
3. The alarm sounder as in claim 1 where the control circuits carry
out an initial calibration process to establish a reference
value.
4. The alarm sounder as in claim 1 where the process is terminated
when a predetermined current value is detected, and, wherein a
local capacitor provides a current flow to drive the class-D
amplifier.
5. The alarm sounder as in claim 4 where the control circuits
monitor a capacitor voltage over a period of time and establish a
current flow therefrom.
6. The alarm sounder as in claim 1 where the control circuits
include volume control circuits coupled to a storage capacitor.
7. The alarm sounder as in claim 6 where a decay time parameter of
the storage capacitor is measured.
8. The alarm sounder as in claim 7 which includes increasing a duty
cycle of a test signal which drives the piezo-electric
transducer.
9. The alarm sounder as in claim 8 which includes increasing a
volume parameter of the test cycle.
10. An alarm sounder comprising a class-D amplifier that drives a
piezoelectric transducer with a variable background monitoring
waveform that is initially set at a first frequency which
substantially exceeds an end of a predetermined audio band and
control circuits that periodically background test the
piezoelectric transducer using the variable background monitoring
waveform when the alarm sounder is not otherwise active to
determine if the alarm sounder is capable of giving an audio alarm
when required thereby incorporating silent testing of the alarm
sounder.
11. The alarm sounder as in claim 10 where the waveform comprises a
square wave.
12. The alarm sounder as in claim 11 which includes a capacitor
where a voltage decay on the capacitor is indicative of the sounder
monitoring current.
13. The alarm sounder as in claim 12 wherein the decaying voltage
is monitored, at least intermittently, to measure an audio
producing current of the alarm sounder.
14. The alarm sounder as in claim 13 where the waveform frequency
is intermittently reduced while continuing to monitor the alarm
sounder audio producing current.
15. A method of monitoring operation of a transducer comprising: a
microcontroller generating a variable frequency square wave; the
microcontroller driving a piezo-electric transducer with the square
wave; the microcontroller monitoring a current of the
piezo-electric transducer as the frequency is being varied; and the
microcontroller determining if the current indicates expected
operation of the piezo-electric transducer thereby incorporating
silent testing of the piezo-electric transducer.
16. The method as in claim 15 where generating includes increasing
a duty cycle parameter thereof.
17. The method as in claim 16 which includes increasing a volume
parameter thereof.
18. The method as in claim 17 where determining includes at least
one of detecting a short circuit, or detecting an open circuit.
19. The method as in claim 16 where generating includes initially
producing the square wave in the vicinity of 20 KHZ.
20. The method as in claim 19 which includes then setting an
amplitude parameter of the square wave to a maximum value.
Description
FIELD
[0001] The application pertains to audible alarm indicating output
devices, or sounders. More particularly, the application pertains
to substantially silent monitoring of alarm sounders.
BACKGROUND
[0002] Modern analogue addressable fire alarm systems use many loop
powered alarm sounders controlled by microcontrollers, to alert
people in protected areas to the presence of a fire alarm
condition. Many alarm sounders use piezo-electric transducers (a
piezo) to reduce the current consumption of the sounders in the
alarm condition. Typically these analogue addressable systems can
continuously monitor all outstation types on each addressable loop
for faults, to ensure the system can be relied on to detect fires
and alert people. In the case of alarm sounders, the actual sounder
output can normally only be switched on and verified during regular
tests with the system in the alarm state.
[0003] While it would be an enormous benefit to continuously verify
that the alarm sounder can actually provide its correct output,
background monitoring has always proved difficult to successfully
implement especially with sounders using a piezo element. In known
systems, complex monitoring waveforms need to be generated, so that
background monitoring is normally only available on speech
variants. However, as a relatively large acoustic output during the
background monitoring has always proved to be unavoidable, its use
in bedrooms for example, is clearly unacceptable.
[0004] One way to guarantee reliable fault detection of the sounder
would be to require that the monitoring frequency be fixed at a
relatively low in-band frequency. This configuration would produce
a monitoring current high enough to provide reliable
discrimination. This however would prevent the monitoring from
being silent, and it would limit its general use.
BRIEF DESCRIPTION OF THE DRAWINGS
[0005] FIG. 1 is a block diagram of an embodiment hereof;
[0006] FIG. 2 is a flow diagram of aspects of operation of the
embodiment of FIG. 1; and
[0007] FIG. 3 is a flow diagram of additional aspects of the method
of FIG. 2.
DETAILED DESCRIPTION
[0008] While disclosed embodiments can take many different forms,
specific embodiments hereof are shown in the drawings and will be
described herein in detail with the understanding that the present
disclosure is to be considered as an exemplification of the
principles hereof, as well as the best mode of practicing same, and
is not intended to limit the claims hereof to the specific
embodiment illustrated.
[0009] Silent monitoring, in accordance herewith can be provided
for a fire alarm piezo-electric sounder that uses a class-D drive
amplifier which normally produces attention tones between 500 Hz
and 1 KHz. Significant higher harmonics are also normally produced
through-out the audio band, and optionally, the sounder may be used
to produce speech messages. The class-D amplifier uses the audio
output providing piezo-electric transducer as a filtering
capacitor. The volume is normally controlled by adjusting the
supply voltage feed to a storage capacitor connected to the
amplifier.
[0010] Silent monitoring can be implemented by periodically
producing a high frequency square wave, starting at a frequency
close to the upper limit of hearing of approximately 20 KHz. The
amplifier is set to the maximum volume, and then the supply rail to
the amplifier is turned off, allowing the amplifier to be energized
by a local storage capacitor coupled to the supply rails.
[0011] Normally the amplifier will draw a small current due to the
piezo being driven at a frequency far outside the speech band and
higher than the upper corner frequency of the amplifier. This also
causes the amplifier to attenuate the monitoring frequency so it
becomes inaudible. The drop in voltage on the storage capacitor
over a fixed period is directly proportional to the current used
and is monitored by local control circuits, which could include a
microcontroller, in the alarm sounder.
[0012] If the piezo element or, the amplifier draws a very high
current, then this is clearly a low impedance type of fault and can
easily be detected by the local control circuits as such. If the
current is lower than expected, then this could be a high impedance
type of fault; however it could also be due to the very high
efficiency of the amplifier, the variation in its impedance at this
high frequency or due to the component variation of the particular
piezo element and storage capacitor used. While all the variations
could be initially calibrated out during manufacturing, it is know
that component values will change with the effects of temperature
and age.
[0013] In one aspect hereof, the alarm sounder generates a square
wave monitoring frequency at descending frequency steps below 20
KHz, with the transition between steps carefully controlled to
minimise the audio content. If an adequate, but not excessive
monitoring current can be detected at any step, then this process
will stop and the test will have been passed, if however the
current is always too small then the test will only stop and fail
at a drive frequency well into the audio band.
[0014] The overall effect of this optimising technique is to
produce a preferred monitoring frequency that will reliably monitor
the sounder and give the lowest possible audio output and therefore
annoyance. If an open circuit type of fault is discovered, the
sounder will actually be tested at its maximum volume and at a
drive frequency which is consistent with the sounder's normal
operation during an alarm. In this case it is a certainty that a
real fault must exists, however even this will be a silent
test.
[0015] FIG. 1 illustrates a block diagram of an alarm/monitoring
apparatus 10 which incorporates silent testing of one or more alarm
sounders in accordance herewith. FIG. 2 and FIG. 3 illustrate
aspects of methods 100, 200 respectively of testing such
sounders.
[0016] Apparatus 10 includes an alarm/monitoring control unit, or
panel 12 of a type generally known to those of skill in the art.
The unit 12 is in bidirectional communication with a plurality 14
of substantially identical alarm sounders 14a, 14b . . . 14i . . .
14n via a medium 16. The medium 16 could be implemented, for
example as an electric cable.
[0017] The unit 12 can communicate information and commands to and
receive information from members of the plurality 14 along with
smoke, fire, or intrusion detectors, without limitation as would be
used in monitoring a region R and providing alarm related
information to individuals in that region.
[0018] Sounder 14i is representative of members of the plurality
14. A description thereof will suffice for the other members of the
plurality 14 as well.
[0019] Sounder 14i is carried by a housing 20 which could be
mounted on a surface in the region R to provide audible alarm
indicating outputs. Sounder 14i receives commands, and other
information along with electrical energy from unit 12 via medium
16. Sounder 14i can also communicate status or test results to the
unit 12 via medium 16 and interface circuits 12a. If desired,
sounder 14i could be in wireless communication with unit 12 and
receive its electrical energy from a local source, without
limitation.
[0020] Housing 20 of sounder 14i carries a programmable control
unit, or microcontroller, 22a along with pre-stored control
software 22b. Housing 20 also carries volume control circuits 24,
storage capacitor 28, monitoring circuitry 30, a class-D amplifier
32 a piezoelectric audible output transducer 34 and an A/C load
36.
[0021] In operation as discussed below, microcontroller 22a
periodically background tests the alarm sounder 14i using a test
signal when the alarm sounder is not active to determine if it is
capable of giving an audio alarm when required. This test signal
starts at an inaudible high initial frequency close to 20 KHz.
First, the quiescent current taken by the class-D amplifier 32, the
monitor circuitry 30 and the hold-up time of the storage capacitor
28 is measured in a calibration test 200 discussed further relative
to FIG. 3.
[0022] The microcontroller 22a sets the volume to 0%, as at 202,
using a PWM control line 40, which drives the volume control
circuit 24. The volume control circuit 24 supplies a voltage supply
level on line 26 to a class-D amplifier 32 and hence controls its
volume. Microcontroller 22a then sets a PWM drive frequency, line
40a, to a very low duty cycle of just less than 0.5%, as at 206, at
the initial start frequency of 20 KHz, as at 204. The audio
envelope generated in this step change is masked by the fact that
the volume is set to 0%.
[0023] The volume is then ramped up to its maximum 100% level, as
at 208, using the PWM control line 40 over a number of seconds, so
that the frequency content of the envelope appearing on the output
of class-D amplifier 32 is too low to be audible. Storage capacitor
28 is now charged up to its maximum voltage, which is equal to the
regulated input voltage 42 obtainable from the medium 16.
Microcontroller 22a now turns off the volume control circuit 24, as
at 210, by setting the PWM control line 40 to 0%.
[0024] Storage capacitor 28 now slowly discharges at a rate
dependent on the actual value of the capacitor 28, the static
circuit loading and the dynamic loading caused by the finite
switching losses of the class-D amplifier 32. The piezoelectric
transducer 34 causes almost no loading because of the very small
duty cycle of the class-D amplifier 326.
[0025] After about a one second discharge period, as at 212,
microcontroller 22a measures the monitor circuitry 30 using an
analogue to digital converter connected to ADC port line 44, as at
214. This calibration reading is termed ADC1 and could be the
result of a number of samples averaged together to filter noise. It
should also be understood that this ADC1 value could also be
checked to see if it is in an expected range, so that many other
hardware faults could be determined.
[0026] With respect to FIG. 2, following on from the calibration
test, of FIG. 3, the duty cycle of the initial test frequency is
slowly increased to 50%, as at 102, to produce a square wave drive
waveform and the volume is also slowly increased to a maximum 100%,
as at 104, by the microcontroller 22a. In both cases the rate of
change is limited so that the frequency content from the class-D
amplifier 32 remains inaudible. The output of the class-D amplifier
32 is now at the maximum drive power for the piezoelectric
transducer 34, for this particular frequency. Microcontroller 22a
now turns off the volume control circuit 24, as at 106, so that
storage capacitor 28 will discharge at a higher rate determined
mainly by the loading from the piezoelectric transducer 34.
[0027] After about one second, as at 108, the microcontroller 22a
measures the monitor circuitry 30 using an analogue to digital
converter connected to the ADC port line 44. This reading is termed
ADC2, as at 110, and may be the result of a number of samples
averaged together.
[0028] A small A/C load 36 may be placed asymmetrically on the
piezoelectric transducer 34 to increase its high frequency loading
and the class-D amplifier 32 may be driven only on the opposite
side of the piezoelectric transducer 34, with a single ended output
38. The A/C load 36 could be just a capacitor, with a value that is
small compared to the capacitance of the piezoelectric
transducer.
[0029] Microcontroller 22a now removes the calibration value ADC1
from the first test measurement ADC2, giving a delta measurement to
determine the load impedance of the alarm sounder. If the delta
measurement is too high, as at 112, it indicates an excessively
high current caused by a low impedance fault, either due to the
piezoelectric transducer 34 or class-D amplifier 32 being faulty.
Microcontroller 22a then reports back to the control panel 12 that
a short-circuit fault exists on the alarm sounder 14i, as at
114.
[0030] If however the delta measurement is too low, it does not
necessarily mean that an open circuit fault has occurred, it could
be that the loading caused by the class-D amplifier 32 driving the
piezoelectric transducer 34 at such a high frequency is just too
small to measure reliably, as the test frequency is far outside the
normal operational range of an alarm sounder, such as 14i.
[0031] If the delta measurement is too low, as at 116, then the
microcontroller 22a goes through a process of reducing the test
frequency by a small amount (say by 1 KHz) and repeating the above
test measurement (as at 104-110) to obtain a new value of ADC2. As
the test frequency moves closer to the normal operational frequency
of the alarm sounder, then the load current taken by the class-D
amplifier 32 due to the piezoelectric transducer 34 must increase
if the alarm sounder is really fault free. The microcontroller 22a
will then finish the silent monitoring when it detects that an open
circuit does not exist. This will be at a frequency that exactly
minimizes the audio output noise and maximizes the robustness of
the measurement.
[0032] If however the delta measurement is too low on each new test
frequency and the test frequency has reached the normal operational
frequency range of the alarm sounder, say for example as low as 3
KHz, then at this minimum frequency it is certain that a real open
circuit fault must exists and the microcontroller 22a will stop the
test and report an open circuit fault to the control panel. Note
that in this condition the monitoring has also remained silent even
while the test frequency is well into the audio band and at its
maximum volume.
[0033] Assuming a successful test of the alarm sounder resulted in
no faults being found, then the square wave test frequency remains
on for about ten seconds, as at 120, so that the storage capacitor
28 is fully discharged i.e. the class-D amplifier 32 falls to 0%
volume, as at 122, before microcontroller 22a ramps the test
frequency off to end the test. This process again ensures that the
frequency content of the audio envelope is masked.
[0034] From the foregoing, it will be observed that numerous
variations and modifications may be effected without departing from
the spirit and scope of the invention. It is to be understood that
no limitation with respect to the specific apparatus illustrated
herein is intended or should be inferred. It is, of course,
intended to cover by the appended claims all such modifications as
fall within the scope of the claims. Further, logic flows depicted
in the figures do not require the particular order shown, or
sequential order, to achieve desirable results. Other steps may be
provided, or steps may be eliminated, from the described flows, and
other components may be add to, or removed from the described
embodiments.
* * * * *